Communications between terrestrial devices such as radios and earth-orbiting satellites are well known. A commercial application of these satellite systems is satellite digital audio radio service (SDARS). SDARS systems broadcast high quality uninterrupted audio through satellites and earth-based stations. SDARS systems typically include an antenna with a low-noise amplifier and a receiver. The antenna initially receives encoded signals from the satellites and/or terrestrial transmitters. The amplifier, which is conventionally housed within the antenna, amplifies the received signal. The receiver decodes the transmitted signal and provides the signal to the radio.
Referring to FIG. 1, a simplified block diagram of a typical satellite digital audio radio service (SDARS) system is shown. An Earth-orbiting satellite 11 broadcasts SDARS signals. The SDARS signals may be received by a SDARS-receiving device 14, such as a radio (shown) or a television (for example), and/or they may be received by stationary transmitters 12. The terrestrial transmitters 12 re-broadcast the SDARS signals, which may then be received by SDARS-receiving devices 14. The SDARS-receiving device 14 includes an antenna (not shown in FIG. 1) to receive the broadcast SDARS signals. Typical SDARS-receiving devices 14 further include other components (not shown), such as an amplifier, receiver, speakers, etc. to convert the SDARS signals into audible sounds and/or visual images.
Terrestrial SDARS-receiving devices 14 commonly use a quadrifilar helix antenna to receive SDARS signals. An exemplary known quadrifilar helix antenna is shown in FIG. 2. The illustrated quadrifilar helix antenna 16 includes four conductive elements 18a-18d, such as electrically-conductive wires, arranged to define two separate helically twisted loops. Each of the loops is connected between an antenna feed and a ground plane, and the conductive elements each fold over itself at a distal point from the antenna feed and the ground plane to form a loop, as shown in FIG. 2. The two conductive elements of a quadrifilar helix antenna 16 are excited in phase quadrature. That is, each conductive element is excited at a 90° phase shift from the adjacent conductive element.
Conventional quadrifilar helix antennas used in SDARS-receiving devices have a number of disadvantages. Known quadrifilar helix antennas are most effective when receiving signals from a satellite at zenith. Known quadrifilar helix antennas are typically less effective at receiving SDARS signals transmitted from low elevation satellites and from stationary terrestrial transmitters. As a result, some SDARS-receiving devices include a second antenna dedicated to receiving SDARS signals from stationary terrestrial transmitters. Further, known quadrifilar helix antennas have limited utility for portable and/or wearable SDARS-receiving devices, such as personal radios, headphones, etc. The interference created by the human body degrades the ability of conventional quadrifilar helix antennas to receive SDARS signals. Moreover, the fact that known quadrifilar helix antennas require a relatively large ground plane makes using such antennas in portable/wearable devices impractical.
The embodiments described below were developed in light of these and other disadvantages of known quadrifilar helix antennas.